Viruses are aetiological agents in a range of diseases in humans and animals, including influenza, mumps, infectious mononucleosis, the common cold, poliomyelitis, measles, german measles, herpes (oral and genital), chickenpox, hepatitis, rabies, warts, cancer and acquired immunodeficiency syndrome (AIDS), to name a few. Viruses range in size from approximately 20-25 nm diameter or less (parvoviridae, picornoviridae) to approximately 200-450 nm maximum dimension (poxviridae), although filamentous viruses may reach lengths of 2000 nm (closterviruses) and can therefore be larger than some bacteria. Viruses lack metabolic machinery of their own and are dependent on their host cells for replication. Therefore, they cannot be grown in synthetic culture media like many other pathogens. Accordingly, specialized approaches are necessary for laboratory diagnosis of viral disease. For example, viruses may be grown in animals, embryonated eggs, or in cell cultures where animal host cells are grown in a synthetic medium and the viruses are then grown in these cells.
Laboratory diagnosis of viral infection is based generally on three approaches: (a) virus isolation, followed by identification (e.g., tissue culture techniques); (b) direct detection of viral components in infected tissues (e.g., by electron microscopy); and (c) demonstration of a significant increase in virus-specific antibodies (e.g., serological techniques). Molecular techniques such as DNA probes or the polymerase chain reaction (PCR) are used for the detection of viruses where cell culture or serological methods are difficult, expensive or unavailable. PCR is also generally the method of choice to detect viral DNA or RNA directly in clinical specimens. The advantage of PCR for viral diagnostics is its high sensitivity; PCR can detect very low numbers of viruses in a small clinical specimen. However, this sensitivity of detection can also cause significant problems in routine viral diagnostics. The significant risk of cross-contamination from sample to sample can outweigh the benefits of detecting small quantities of a target viral nucleic acid. Cross-contamination can also result in false positives, making interpretation of epidemiological data impossible.
Flow sorting devices have been used to analyze and separate larger biological materials, such as biological cells. Conventional flow sorters, such as FACS have numerous problems that render them impractical for analyzing and sorting viruses and other similarly sized particles. FACS and other conventional flow sorters are designed to have a flow chamber with a nozzle and use the principle of hydrodynamic focusing with sheath flow to separate or sort material such as biological cells (1-6). In addition, most sorting instruments combine the technology of ink-jet writing and the effect of gravity to achieve a high sorting rate of droplet generation and electrical charging (7-9).
Despite these advances, many failures of these instruments are due to problems in the flow chamber. For example, orifice clogging, particle absorption and contamination in the tubing may cause turbulent flow in the jet stream. These problems contribute to the great variation in illumination and detection in conventional FACS devices. Another major problem, known as sample carryover, occurs when remnants of previous specimens left in the channel back-flush into the new sample stream during consecutive runs. A potentially more serious problem occurs when dyes remain on the tubing and the chamber, which may give false signals to the fluorescence detection or light scattering apparatus. Although such systems can be sterilized between runs, the procedure is costly, time consuming, inefficient and results in hours of machine down time.
In addition, each cell, as it passes through the orifice, may generate a different perturbation in response to droplet formation. Larger cells can possibly change the droplet size, non-spherical cells tend to align with the long axis parallel to the flow axis, and deformable cells may elongate in the direction of the flow (8, 9). This can result in some variation in the time from the analysis to the actual sorting event. Furthermore, a number of technical problems make it difficult to generate identically charged droplets, which increases deflection error. A charged droplet may cause the next droplet of the opposite polarity to have a reduced charge. On the other hand, if consecutive droplets are charged identically, then the first droplet might have a lower potential than the second droplet, and so on. However, charged droplets will have a defined trajectory only if they are charged identically. In addition, increasing droplet charges may cause mutual electrostatic repulsion between adjacent droplets, which also increases deflection error. Other factors, such as the very high cost for even modest conventional FACS equipment, the high cost of maintenance, and the requirement for trained personnel to operate and maintain the equipment further hinder the widespread accessibility and use of this technology.
Flow cytometry has also been used to separate biological cells. For example, Harrison et al. (38) disclose a microfluidic device that manipulates and stops the flow of fluid through a microfabricated chip so that a cell can be observed after it interacts with a chemical agent. The cells and the chemical agent are loaded into the device via two different inlet channels, which intersect with a main flow path. The flow of the fluid is controlled by a pressure pump or by electric fields (electrophoretic or electro-osmotic) and can be stopped so that the cells can be observed after they mix and interact with the reagent. The cells then pass through the main flow pathway, which terminates through a common waste chamber. Harrison et al. do not, however, provide a device or methods for sorting cells or other biological materials, nor do they suggest or motivate one having ordinary skill in the art to make and use any such device.
For reasons of sensitivity, flow cytometry has by and large been limited to the analysis of cells. Although it is marginally possible to observe light scatter directly from large viruses, this strains the detection limit for conventional flow cytometry. The practical limit of detection for these traditional methods is a spherical particle no smaller than 150 nm, which excludes many viruses (8). The development of flow cytometric techniques for the sorting of viruses is also plagued by other problems related to the size of virus particles. Their small size results in a high diffusion constant making them difficult to control by sheath flow. Containment of the viruses is also important during any flow cytometry sorting process because extruding droplets containing viruses presents a potential biohazard.